Kinetis enet: buffer management update

This patch manages the packet buffer used by the upper layers by making sure it is always set to a valid transmit buffer that can be used by the MAC-NET core.  The only exception to this is when the upper layer re-uses a receive buffer to send a response. In this case, the updated receive buffer is swapped with an empty transmit buffer.  If there is no empty transmit buffer available, the packet will be dropped.

Signed-off-by: Andrew Webster <awebster@arcx.com>

Kinetis enet: add support for DBSWP

Signed-off-by: Andrew Webster <awebster@arcx.com>